1. Field of the Invention
The present invention relates generally to implantable medical devices and more specifically relates to urinary injection stents.
2. Description of the Prior Art
It has been known for some time to treat a patient having a partially or fully occluded ureter with a stent to permit urine flow from the kidney(s) to the bladder. This occlusion may result from infection, spurious tissue growth, or inorganic deposit. An interesting, but not widely accepted, approach to the problem is found in U.S. Pat. No. 4,307,723 issued to Finney. This technique employs an essentially solid rod with external grooves for the passage of the fluid. By far the most accepted approach is to employ an implantable stent having an interior lumen for the passage of the fluid.
One of the problems addressed in the art is associated with the length of the ureter which varies from patient to patient. U.S. Pat. No. 4,790,810 issued to Pugh, Jr. et al; U.S. Pat. No. 4,643,716 issued to Drach; and U.S. Pat. No. 4,931,037 issued to Wetterman each provide a device having a stent with a length which may be adjusted by trimming at the time of manufacture or intraoperatively. U.S. Pat. No. 4,813,925 issued to Anderson, Jr. et al. suggests a stent which automatically adjusts to the length of the ureter by employing a helical shape along the body of the stent.
For most patients, the stent is implanted for a period of several weeks during which the source of the occlusion is treated. After the occlusion has been successfully abated, the stent is no longer needed and is removed from the patient. It is most desirable to remove the stent without another surgical procedure. Therefore, most present day stents employ a tether whereby the stent may be removed without more traumatic intervention. U.S. Pat. No. 4,671,795 issued to Mulchin shows a stent having the tether attached to the center of the stent rather than to the distal end. Removal of the stent using magnetic attraction is discussed in U.S. Pat. No. 4,790,809 issued to Kuntz.
To be effective, the stent must employ means for preventing migration from the implanted position. The most accepted technique is to preform the proximal and distal ends into shapes which tend to retard migration. The three most popular shapes are the helix as seen in U.S. Pat. No. 4,531,933 issued to Norton et al; the hook as shown in U.S. Pat. No. 4,874,360 issued to Goldberg et al; and the "pigtail" as employed at the proximal end of U.S. Pat. No. 4,950,228 issued to Knapp, Jr. et al. During implantation, the preformed regions are straightened through the use of a guide wire or guide catheter. Upon removal of the guiding device, the stent tends to assume its stable shape thereby preventing migration.
The implantation procedure is normally performed by advancing the stent through the urethra and the bladder into the ureter and kidney. Various early stents had an internal central lumen with a closed proximal end, such as shown in U.S. Pat. No. 4,713,049 issued to Carter. The difficulty with closed end stents is that the much smaller diameter guide wire cannot be advanced into position first but requires the initial penetration to be made with the proximal tip of the much larger diameter stent.
U.S. Pat. No. 4,610,657 issued to Densow partially addresses this difficulty by providing an open, but tapered proximal tip. This permits a small diameter guide wire to extend beyond the proximal tip of the stent whereas a larger diameter guide wire may be used to push the stent to advance it into position. This is not a complete solution, however, because it does not permit the implanting physician to advance the guide wire into position and then simply push the stent into position over it. Through the use of a complex two piece guide wire/catheter assembly, U.S. Pat. No. 4,787,884 issued to Goldberg permits the smaller diameter inner guide wire to be advanced slightly ahead of the stent. However, this technique does not permit completely advancing the guide wire before the stent enters the patient's body.
An approach which makes the implantation easier for the physician permits complete advancement of the guide wire before advancement of the stent. This is best accomplished by pushing the stent into position from its distal end using a guide catheter which slides easily over the guide wire. To accomplish this, however, it is desirable to have sufficient attachment at the distal end of the stent to transfer the necessary torque to ensure proper placement. U.S. Pat. No. 4,963,129 issued to Rusch uses a complex mechanism to interlock the proximal end of the push catheter and the distal end of the stent. Another equally complex approach is used in U.S. Pat. No. 4,957,479 issued to Roemer.
U.S. Pat. No. 4,913,683 issued to Gregory shows a stent system which is also useful for injecting a dye or other fluid. According to this design, the distal end of the stent is flared to permit engagement by the proximal tip of a pushing catheter. Unfortunately, this produces substantial strain on the distal end of the stent and therefore, restricts the types and thicknesses of material which may be used to fabricate the stent. This abutting push design also provides undesirable handling characteristics by severely limiting torque transmission from the push catheter to the stent. The Gregory design does, however, provide for the infusion of a dye or other liquid.